This invention relates to an integrated circuit testing apparatus and more specifically to a probe unit and a method of making the probe unit for the testing apparatus.
Integrated circuits are fabricated on a semiconductor wafer using a variety of chemical and physical processes. Several hundred integrated circuits may be fabricated on a single wafer and sectioned into individual pieces after the integrated circuits have been tested for functional integrity.
Integrated circuits are tested by contacting the conductive pads on the wafer with a probe unit, which is connected to a probe card. The probe card contains the circuitry necessary to perform electrical diagnosis and characterization of the integrated circuits. A traditional probe unit includes pogo pins or needles suspended from the body of the probe unit for contacting the electrical pads of the integrated circuits of the wafer. The pins or needles are conductive, and thus when in proper contact with the pads provide electrical communication between an integrated circuit being tested and the probe card. Providing adequate contact between the pins or the needles and the conductive pads, however, is not trivial because the conductive pads on the substrate can have varying height due to manufacturing tolerances. Thus when the probe unit is placed in contact with the wafer, one pin or needle can be advanced too far and scratch or damage the pad of the device with which it makes contact, while another pin or needle does not even contact it""s respective pad. Although the pogo pins or needles are suspended from the body of the probe unit using a spring mechanism to compensate for some height differences in the contact pads, this contact problem persists.
An alternative to a traditional probe unit is a flexible probe unit which compensates for the differences in height between the contact pads of a wafer. U.S. Pat. No. 4,906,920 (""920) describes an example of a flexible probe unit. The ""920 probe unit includes a flexible membrane supporting contact bumps. The flexible membrane pivots horizontally to adjust for non-planarity of the contact pads on a wafer. The contact bumps themselves, however, are not flexible. Therefore, compliance of the probe unit is limited.
An objective of the present invention is to provide a probe unit for an integrated circuit testing apparatus which includes flexible contacts that individually compensate for any differences in height or shape between the pads.
The invention relates to an integrated circuit testing apparatus. The apparatus includes a probe card, and a probe unit. The probe unit includes a plurality of conductive elastic bumps and a plurality of conductors positioned to conduct signals from the bumps to the probe card. In one embodiment, the apparatus further comprises a substrate disposed between the probe card and the probe unit and a flexible member disposed adjacent the substrate. The substrate can comprise a ceramic or a semiconductor substrate. The flexible member can be an elastomer.
In one embodiment, the conductive elastic bumps comprise a conductive elastic polymer. The conductive elastic polymer can be a conductive polymer doped with a plasticizer. The conductive polymer can be selected from a group consisting of polyaniline, polypyrrole, polythiophene, polyphenylene, polyacetylene and their derivatives. The conductive polymer can comprise polyaniline, and the plasticizer can comprise an acid. The acid can be one of dodecylbenzenesulfonic acid and camphorsulfonic acid. In another embodiment, the conductive elastic polymer comprises a surface modified conductive polymer. In one detailed embodiment, the conductive elastic polymer comprises polyaniline graft copolymerized with a vinyl monomer. The vinyl monomer can be selected from a group consisting of acrylic acid, sodium salt of styrenesulfonic acid, acrylamide, and 4-vinylpyridine. In another embodiment, the conductive elastic polymer comprises a conductive polymer cross-linked with a resin-forming adhesive copolymer. The conductive elastic polymer can comprise polyaniline formaldehyde resin. For example, the conductive elastic bumps can comprise a polyaniline cross-linked with a copolymer comprising acrylic acid and acrylamide. In still another embodiment, the conductive elastic polymer comprises an electrically conductive polymer copolymerized with a cross-linked dielectric polymeric backbone.
In one embodiment, the conductive elastic polymer is cross-linked. In another embodiment, the conductive elastic polymer is an interpenetrating polymer. The plurality of conductive elastic bumps can be hemispherical in shape. The conductive elastic bumps are capable of being compressed to less than about 70% of the original thickness. The conductive elastic bumps can comprise a conductive polymer layer on an outer surface.
In one embodiment, the probe card is in electrical communication with the plurality of conductive elastic bumps through a via which extends through a substrate and a PCB supporting the bumps. Where the probe unit includes a flexible member disposed adjacent the substrate, a flexible conductor is disposed within the flexible member for communication with the via. The flexible conductor can be a gold fuzz ball.
In another embodiment, the probe unit further comprises a plurality of ground pads disposed between a plurality of striplines and the bumps, where each stripline is connected to a respective bump. The plurality of conductive elastic bumps are individually disposed on a respective microstripline. In one detailed embodiment, one of the plurality of striplines forms a laser trimmable flange of a waveguide. In another detailed embodiment, the probe unit further comprises active or passive components embedded within the probe unit. The passive component can be a capacitor, and an active component can be a multiplexer.
In still another embodiment, the probe unit comprises a plurality of independently mobile sections, each section comprising at least one contact bump. A flexible member can support the plurality of sections. An elastomer can be disposed within a space between adjacent sections.
In yet another embodiment, the probe unit further comprises a voltage equilibrating site surrounding a contact bump. The voltage equilibrating site includes a first dielectric ring surrounding the contact bump, a metal ring surrounding the first dielectric ring, and a second dielectric ring surrounding the metal ring. The voltage equilibrating sites may be used to clean the bumps. In another embodiment, the probe unit further comprises a temperature sensor.
The invention also relates to a method of manufacturing a probe for an integrated circuit testing apparatus. The method includes the steps of: a) providing a printed circuit board (PCB); b) forming an conductive elastic material; and c) disposing the conductive elastic material on a surface of the PCB to form a plurality of bumps. In one embodiment, step b) comprises forming an conductive elastic polymer. An conductive elastic polymer in one embodiment is formed by doping a conductive polymer with a plasticizer. Alternatively, an conductive elastic polymer can be formed by modifying the surface of a conductive polymer. For example, a surface of a conductive polymer can be modified by graft copolymerizing with a vinyl monomer. Examples of suitable vinyl monomers include acrylic acid, sodium salt of 4 styrenesulfonic acid, acrylamide, and 4-vinylpyridine. The conductive elastic polymer can be a cross-linked polymer. One example of a suitable conductive elastic polymer is a polyaniline-dodecylbenzenesulfonic acid gel. Another example of a suitable conductive elastic polymer is a copolymer resin formed by preparing aniline-formaldehye condensate and polymerizing aniline using the condensate.
In one embodiment, step c) comprises printing the gel on the surface of the PCB. In another embodiment, step c) comprises dispersing a copolymer resin in an organic solvent on the surface of the PCB. The dispersion can comprise a conductive polymer in an organic solvent containing a photocross-linkable agent. In still another embodiment, step b) comprises forming a precursor of a conductive elastic material, and step c) comprises disposing the precursor on a surface of the PCB, patterning the precursor, and curing the precursor to form a plurality bumps. Curing the precursor cross-links the conductive elastic polymer. Curing the precursor comprises exposing the precursor to UV radiation. The precursor can be partially cured before being patterning, and subsequently fully cured to form a plurality of bumps. In one example, the precursor comprises polyaniline and a photocross-linkable adhesive copolymer.
The formed bumps can be further treated in a variety ways. For example, a conductive polymer layer can be placed on a surface of the conductive elastic polymer. The bumps can also undergo a planarizing step by contacting the bumps with a planar surface, while the bumps are fully cured. In one example, a plurality of elongated bumps are first formed on a PCB, and the bumps become spherical during the planarization step. The bumps can also be polished using silver or diamond. The bumps can be passivated with a chemical agent such as sulfur.
In another embodiment, the method further includes the step of forming a variety of devices inside the probe. For example, a laser trimmable flange of a waveguide can be formed within the probe such that each waveguide is connected to a respective bump. These waveguides can be formed by laser ablating the flange to tune an impedance of the wave guide. The waveguide can handle a frequency up to about 20 GHz. An active component such as a multiplexer or a passive component such as a capacitor can be formed within the probe.
In still another embodiment, the method further comprises the step of disposing a substrate on a surface of the PCB away from the surface having the bumps. The substrate can be a ceramic or a semiconductor substrate.
The invention also relates to a method of testing an integrated circuit. The method includes the steps of: a) providing a substrate having the integrated circuit; b) contacting a plurality of contact pads on the substrate with a plurality of conductive elastic bumps of a probe unit which is in communication with a probe card; and c) measuring an electrical property of the integrated circuit. In one embodiment, the method further includes the step of applying a pressure in the range from 0.1 grams to 0.5 grams per contact bump to maintain less than 300 ohms of contact resistance, in order to contact the plurality of contact pads with the plurality of bumps. In another embodiment, a flexible member for providing additional compliance of the probe unit is disposed between the probe unit and the probe card.
In still another embodiment, the method further comprises the step of making a plurality of independently mobile sections. Each section comprises at least one bump disposed on a PCB, and a flexible member supports the plurality of sections. An elastomer can be placed in the spaces between adjacent sections.
In still another embodiment, the method further includes the,step of forming a voltage equilibrating site around each bump. A voltage equilibrating site, for example, has a first dielectric ring around a bump, a metal ring around the first dielectric ring, and a second dielectric ring around the metal ring. This probe unit can sense voltage accumulation on the substrate, and equalize the voltage between the probe unit and the substrate.
In yet another embodiment, the method further includes the step of cleaning the bumps. The steps of cleaning the bumps include applying various electrostatic forces around the bumps to attract particles away from the bumps.